1. Field of the Invention
This invention relates to a catalyst for purifying an exhaust gas discharged mainly from a diesel engine and a process for purifying the exhaust gas using the catalyst. More specifically, this invention relates to a catalyst capable of efficiently purifying nitrogen oxides (NOx), which are generally difficult of purification in an excess oxygen atmosphere, and manifesting durability, and a process for purifying NOx using the catalyst.
2. Description of Related Art
In the global trend toward rising consciousness of environmental conservation and strengthening emission control, cars equipped with a diesel engine of high fuel efficiency have been reevaluated throughout the world particularly in Europe. The reduction of NOx emission which is one of the harmful components in the exhaust gas of diesel engines has turned into a subject.
In an oxygen rich environment like the exhaust gas of diesel engines, the decrease of NOx is difficult because an oxidation reaction proceeds on the catalyst. Thus, a technique which promotes reduction of NOx utilizing the unburnt hydrocarbons in the exhaust gases or the fuel as a reductant has been examined.
Since the temperature of exhaust gases from the diesel engine is low as compared with that from the gasoline engine, a noble metal, predominantly platinum, which begins to manifest reactivity at a low temperature in an oxidizing atmosphere (excess oxygen atmosphere), is adopted for decreasing hydrocarbons and carbon monoxide. Meanwhile, NOx reduction catalyst which uses hydrocarbon as the reductant, platinum has been cited as a catalytically active component. In such circumstances, many catalysts composed of zeolite and platinum have been proposed as described, for example, in U.S. Pat. No. 5,330,732, U.S. Pat. No. 5,762,892, JP-A-10-156144, and JP-A-11-90235.